Catalytic cracking has been shown to improve the quality of pyrolysis oils generated from a variety of high oxygen content feedstocks including rice husks, rice straw (Chen et al., (2003) Energy Conversion & Management 44: 1875-1884), pine wood (Carlson et al., (2011) Energy Environ. Sci. 4: 145-161; Chen et al., (2003) Energy Conversion and Management 44: 1875-1884; Valle et al., (2007). Int. Chem. Reactor Engineering 5: 1-10), maple wood (Adjaye & Bakhshi (1995) Fuel Processing Technol. 45: 185-202; Adjaye & Bakhshi (1995) Fuel Processing Technol. 45: 161-183), and poplar wood (Lu et al., (2010) Fuel 89: 2096-2103).
The cracking process was developed by the petroleum industry to crack and rearrange high boiling, high molecular weight petroleum crude oil fractions to yield predominantly gasoline and other light hydrocarbons (Corma et al., (2007) J. Catalysis 247: 302-327). Cracking catalysts generally used for treating biomass-derived oils have been acidic zeolites with ion exchange capacity and size selectivity functionality. They have also been shown to effectively deoxygenate bio-oil feedstocks and form desirable end-products, including small alkanes and aromatics (Park et al., (2010) Appl. Catalysis B: Environmental 95: 365-373; Corma et al., (2007) J. Catalysis 247: 302-327; Adjaye & Bakhshi (1995) Fuel Processing Technol. 45: 185-202).
Catalyst coking, however, is a significant problem that progressively reduces the effectiveness of the catalyst. Many studies have sought methods to reduce coke formation (Corma et al., (2007) J. Catalysis 247: 302-327; Valle et al., (2007). Int. Chem. Reactor Engineering 5: 1-10; Elliott & Neuenschwander (1996) in: Bridgwater & Boocock (Eds.), Developments in Thermochemical Biomass Conversion. Blackie Academic & Professional, London, Vol. 1, pp. 611-621). Adjaye and Bakhshi (Adjaye & Bakhshi (1995) Fuel Processing Technol. 45: 185-202; 161-183) attempted to catalytically crack fast pyrolysis bio-oil with five different catalysts (ZSM-5, H-Y-zeolite, H-mordenite, silicalite, and silica-alumina). While the yield of organic liquid product was highest with ZSM-5 (34% w/w of feed), the coke yield was also significant at 20-29 wt %. If coking can be reduced, catalytic upgrading using acid zeolites would become an economically viable method to produce hydrocarbon fuels from lignocellulosic feedstocks.
One way to minimize coke formation on zeolite catalysts is to remove coke precursors from the oil prior to cracking upgrading. Compounds that are considered to promote coke formation include aldehydes, oxyphenols, furfural, and lignin-derived oligomers (Gagnon & Kaliaguine (1988) Ind. Eng. Chem. Res. 27: 1783-1788; Lu et al., (2010) Fuel 89: 2096-2103). Other attempted methods have involved hydrotreating of bio-oil (Gagnon & Kaliaguine (1988) Ind. Eng. Chem. Res. 27: 1783-1788; Laurent & Delmon (1994) Applied Catalysis A: General 109: 77-96; Centeno et al., (1995) J. Catalysis 154: 288-298; Wildschut et al., (2009) Ind. Eng. Chem. Res. 48: 10324-10334). The studies have indicated that the carbohydrate fraction is a major contributor to coke formation. There is, however, currently little research that attempts to remove coke precursors from bio-oil before upgrading.